Hey Trevor

[mlp141]

Hey Trev -- what should I use for break fluid? I am not sure what's in the truck now. Can I mix? I read online somewhere that Dot 5 is needed.

Please advise.

Thanks.

[siiirhd88]

I'm no Trevor, but whatever brake fluid you use needs to be compatible with what you are currently using. Dot 3 and 4 are generally chemically compatible with each other, the major difference being the boiling point of the fluid. Dot 5 is a silicone based fluid, and is not compatible with the Dot 3 or 4 fluids. Dot 5 can be used on a new or rebuilt and flushed system, and if your truck currently uses Dot 5 you need to keep using it.

The Brits used to use a different rubber compound in their brake and clutch seals that required use of the special British brake fluids, as made by Girling and Castrol. At least since the '80s the Brit rubber has been made to work with any of the common brake fluids, but I still tend to use the Castrol LMA brand of fluid. The newer Castrol LMA is branded as a 'synthetic' fluid, but I think the fluid is not really any different than the old LMA fluid. The LMA stands for 'low moisture absorbtion' but all Dot 3 and 4 brake fluids are hygroscopic and absorb moisture readily. The moisture lowers the boiling point of the fluid, leading to brake performance issues and also allows for system corrosion. The brake and clutch fluids should be flushed and replaced as needed to maintain optimum system performance. I tend to replace the fluid when it discolors.

The Dot 5 silicone fluid is not hygroscopic but any water in the system tends to collect into bubbles, which can accumulate causing brake issues.

If you do a google search there is lots of info out there, but the bottom line is for an existing system you need to use a fluid that is compatible with what is in there. If you rebuild and start from a clean and dry system, or one that has been flushed to remove the old Dot 3 of 4 fluids, then Dot 5 silicone could be used.

Bob

[rvrsrvc]

Bob nailed it. I've had less than stellar experiences with the DOT5 silicone brake fluid, and other than race cars or Rolls Royce, I use Castrol GT/LMA exclusively.

[mlp141]

Thanks guys.

The fluid in the resovior looks pretty clean, but I have no idea if its DOT 3 or 4. Is there a way to tell?

[Tom109]

I've had less than stellar experiences with the DOT5 silicone brake fluid...

Care to elaborate?

Tom P.

[siiirhd88]

If you don't have the container, there is no good way to determine if the fluid is Dot 3 or Dot 4. Dot 4 is the newer type fluid, with a slightly higher boiling point than Dot 3. Both types eat paint, absorb moisture, and taste about the same.....

Dot 3 and Dot 4 are supposed to be compatible. Mixing Dot 3 into a Dot 4 system should cause no problems, but the boiling point will be a tad lower. In a Series brake system with drum brakes, the boiling point of the fluid should be of no consequence. Excessive drum brake use usually results in a hard pedal and brake fade due to overheated linings. Disc brakes can generate much more heat and direct it into the caliper fluid space adjacent to the pad. This heat can boil the brake fluid in the caliper, causing a gas void to form resulting in a soft pedal or a pedal that can go to the floor before the void space can refill. The normal pedal feel usually returns as the caliper cools and the gas void condenses. Keeping the fluid clean and as free of moisture as possible helps.

Find a fluid that you like and can readily obtain and stick with it. Keep the lid on the container and ensure the cap seal is in good condition during storage after opening. It only takes a small amount of moisture to effect the fluid and this is nearly impossible to detect. If the fluid appears milky, it has absorbed way too much moisture and should not be used if in a container, or flushed if in a system. Dark colored fluid is usually from corrosion products, again caused by moisture, or by the seal rubber beginning to slowly break down.

I've used Castrol LMA since the '70s. It can be difficult to find, and in the York area only Pep Boys keeps it in stock.

Bob


Thanks guys.

The fluid in the resovior looks pretty clean, but I have no idea if its DOT 3 or 4. Is there a way to tell?